Geochemistry, Geothermobarometry and Geochronology of High-Pressure Granulites and Implications for the Exhumation History of Ultrahigh-Pressure Terranes: Dulan, Western China

Benjamin David Joseph Christensen
September 2011

Abstract

The Dulan ultrahigh-pressure (UHP) rocks of the North Qaidam terrane, Western China, represent continental crust that has been subducted ~100 km during continental collision. Adjacent granulites representing burial to ~50 km could either be overprinted eclogites or a separate high-pressure-high-temperature (HP-HT) granulite unit. Overprinted eclogites and HP-HT granulites imply different P-T-t paths for UHP rocks. Metamorphic conditions for the granulites are 750–880 °C and 14–17 kbar. Zircon U-Pb geochronology, REE patterns and Ti-in-zircon thermometry indicate an increase in temperature from ~800 °C (449 Ma) to ~900 °C (418 Ma). This temperature increase could explain the presence of granulite leucosomes and tonalites (419 Ma) interpreted to represent partial melts. The temperature increase coincides with the end of UHP metamorphism and may be related to exhumation of UHP rocks. These granulites represent a separate HP-HT unit from the thickened overriding crust in a continental collision zone, above subducting UHP rocks.

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